Electrographic and electrophotographic processes form images on selected receivers, typically paper, using small dry colored particles called toner. The toner usually comprises a thermoplastic resin binder, dye or pigment colorants, charge control additives, cleaning aids, fuser release additives, and optionally flow control and tribocharging control surface treatment additives. The thermoplastic toner is typically attached to a print receiver by a combination of heating and pressure using a fusing subassembly that partially melts the toner into the fibers at the surface of the receiver.
Typically, in an electrographic or electrophotographic printer or copier (collectively referred to herein as “printers”), a heated fuser roller/pressure roller nip is used to attach and control the toner image to a receiver. Heat can be applied to the fusing rollers by a resistance heater, such as a halogen lamp. And, it can be applied to the inside of at least one hollow roller and/or to the surface of at least one roller. At least one of the rollers in the heated roller fusing assembly is usually compliant, and when the rollers of the heated roller fusing assembly are pressed together under pressure, the compliant roller then deflects to form a fusing nip.
Most heat transfer between the surface of the fusing roller and the toner occurs in the fusing nip. In order to minimize “offset,” which generally refers to the amount of toner that adheres to the surface of the fuser roller, release oil is typically applied to the surface of the fuser roller. Release oil is generally made of silicone oil plus additives that improve the attachment of the release oil to the surface of the fuser roller and that also dissipate static charge buildup on the fuser rollers or fused prints. During imaging, some of the release oil attaches to the imaged and background areas of the fused prints.
The toner image resident on the surface of the imaging member, such as a photosensitive member or dielectric insulating member, may be transferred to a receiver material using a variety of different methods. For example, the transfer may be a direct transfer to the receiver material. Alternatively, the transfer may be an intermediate transfer in which toner is first transferred to an intermediate transfer medium and then transferred a second time in a second transfer station to the final receiver material. Other methods might also be used.
Various printers might have different printing capabilities depending on their design and their particular operational configurations. For example, different printers might have different imaging speeds. Some printers might be designed for low-capacity use and therefore might only be capable of imaging a relatively small number of pages within a given amount of time. Other printers, however, might be designed for high-capacity use and therefore might be capable of imaging a relatively large number of pages within the same amount of time.
In another example of differing print capabilities, some printers might only be capable of printing on a single side of a receiver material. Printing on a single side of a receiver medium is oftentimes referred to as simplex printing. Other printers might be capable of printing on both sides of a receiver material, which is oftentimes referred to as duplex printing. Duplex printing may be used in a variety of different applications, such as commercial printing applications and other high-volume applications. However, it might also be used in low-volume applications and non-commercial applications.
Conventional duplex imaging systems, however, may have various disadvantages. For example, many conventional duplex imaging systems require the receiver to pass through the system multiple times. U.S. Pat. No. 4,095,979 teaches transferring a first image to a first side of a copy sheet, inverting the copy sheet while the first image thereon remains unfixed, transferring the second unfixed image to the second side of the copy sheet, and then transporting the copy sheet with the first and second unfixed images to a fixing station.
U.S. Pat. Nos. 4,191,465, 4,212,529, 4,214,831, 4,477,176, 5,070,369, 5,070,371, 5,070,372, and 5,799,236 all teach the use of inverters, turn around drums, turn over stations and the like that require a receiver to make multiple passes through the system in order to image on both sides of the receiver. These systems, and others like them, require special handling of the receiver, which can reduce the speed with which the systems can perform duplex imaging.
U.S. Pat. Nos. 5,799,226, 5,826,143, 5,899,611, 5,905,931, 5,970,277, 5,930,572, 5,991,563, and 6,038,410 generally pertain to an apparatus in which a single photoconductor carrying a toner image comes into contact with a single intermediate transfer belt and transfers the image to the intermediate transfer belt at a first transfer station using a corona device. The intermediate transfer belt temporarily holds the first image and transports it in a similar fashion to permit the transfer of a second image from the photoconductor to the top side of a receiver sheet at a first transfer station.
The belt then carries the receiver sheet with the top side image to a second transfer station at which the first image on the intermediate transfer belt is transferred to the bottom side of the receiver sheet. The receiver sheet with duplex images is then transported to a fixing station. Because the intermediate transfer belt temporarily holds the first image for a period of time representing one cycle of the intermediate transfer belt, the speed with which these systems can perform duplex imaging may also be limited. This can be disadvantageous for high-volume and high-speed imaging applications.
Directed aerosol toner development, in the general field of direct electrostatic printing, is an alternative to traditional electrophotographic systems. In directed aerosol toner development, a photoconductor is not required for image formation. Toner in the state of an airborne aerosol may be directed in an image-wise fashion to the surface of an insulating dielectric surface. Alternatively, a real image of toner particles may be written directly on a suitable recording medium. The real toner image is then formed without the need for the charging and exposure steps used in conventional electrophotographic systems.
In addition, a simpler dielectric medium can be used to receive charged particles directly comprising a latent image of charges on the surface. Charged particles include, for example, ions (e.g., cations and anions), dry toner (e.g., electrophotographic and electrographically applied powder paint) and liquid toners (e.g., aqueous, non-aqueous, organic, inorganic, and inks). These are merely examples, and other charged particles might be used.
Light is generally not required in direct electrostatic printing systems, and therefore they also generally do not require the optical sub-systems that are used in conventional electrophotographic systems. In direct electrostatic printing, an aperture array print head system can be used to directly create either a latent image of charged ions that can be subsequently developed with toner material or to directly create a real image of toner particles. Such systems are described in various U.S. patents; however, these systems suffer from many of the same process speed and other disadvantages as electrophotographic systems when performing duplex imaging.
Therefore, there exists a need for improved systems for duplex imaging.